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Josh Schaefferkoetter [email protected] Spintronics Introduction Conventional electronic devices rely on the transport of the electrical charge of electrons in a semiconductor such as silicon. Currently, however, physicists are trying to exploit the 'spin' of the electron rather than its charge to create a remarkable new generation of 'spintronic' devices which will be smaller, more versatile and more robust than those currently making up silicon chips and circuit elements. Magnetoelectronics, Spin Electronics, and Spintronics are different names for the same thing: the use of electrons' spins in information circuits. Fundamental Advantages All spintronic devices are based on the same principles. Information is stored into spins as one of two possible spin orientations. The spins, being attached to mobile electrons, carry the information along a conductor, most likely a wire. Then the information is read at a terminal. Spin orientation of conduction electrons survives for a relatively long time, on the order of nanoseconds, compared to femptoseconds for electron momentum. This makes spintronic devices particularly attractive prospects for memory storage and magnetic sensors applications. Further, this introduces the possibility of using the physical up and down spin orientations as bits of information in a quantum computer. In 1965, Gordon E. Moore wrote, The complexity for minimum component costs has increased at a rate of roughly a factor of two per year ... Certainly over the short term this rate can be expected to continue, if not to increase. Over the longer term, the rate of increase is a bit more uncertain, although there is no reason to believe it will not remain nearly constant for at least 10 years. That means by 1975, the number of components per integrated circuit for minimum cost will be 65,000. I believe that such a large circuit can be built on a single wafer.1 This empirical observation came to be known as Moore’s Law. It has been formulated to suggest that, for minimum components cost, the number of transistors on an integrated circuit will grow exponentially, more or less doubling every eighteen months. As the electronics industry strives for miniaturization of devices and components, the quantum effects of the wave-like behavior of the electrons become a looming problem that is no longer ignorable. Although the trend of charge transport within semiconductors will continue for the next few years, some experts predict that silicon technology is beginning to approach its fundamental limits. For example, by 2008 the width of the electrodes in a silicon microprocessor is predicted to be just 45 nanometers across. 2 Obviously this will place severe demands on the materials and manufacturing techniques used in the semiconductor industry. New technology has been proposed which would involve a complete set of new materials, new handling and processing techniques, and altered circuit design. Such developments include single-electron transistors and molecular-electronic devices based on organic materials or carbon nanotubes. The ability to exploit the spin degree of freedom in semiconductors promises new logic devices with enhanced functionality, higher speeds and reduced power consumption, and since these devices could be made with many of the tools already used in the electronics industry, development would be considerably faster than other alternatives. Underlying Pysics The primary requirement of a spintronic device is to have a system that can generate a current of spin-polarized electrons; the simplest method is to pass the current through a ferromagnetic material. Also needed is a system that is sensitive to the spin polarization of the electrons and thus interpret the transported information. Many devices also have a stage in between that changes the current of electrons depending on the spin states. GMR A common application of ferromagnetic spin polarization is the giant magnetoresistance device. A typical GMR device consists of at least two layers of ferromagnetic materials separated by a spacer layer. When the two magnetization vectors of the ferromagnetic layers are aligned, then an electrical current will flow freely, whereas if the magnetization vectors are antiparallel, then the resistance of the system is higher. There are typically two types of GMR device, where the current flows parallel (CIP) and perpendicular (CPP) to the interfaces between the different layers. While the majority of GMR devices are of type CIP, the CPP version is not only easier to analyze theoretically, but also relates the physics to the tunneling magnetoresistance effect.8 TMR combines spin the asymmetry of the population of the two spin channels in the ferromagnetic materials and the quantum tunnel effect. This effect occurs when two ferromagnetic layers are separated by a thin insulator; the resistance of the tunneling current changes with the relative orientation of the two magnetic layers. TMR has possible applications in MRAM development and replacing conventional spin valves in HD reading heads. Spin Valve This brings us to an appropriate point to address the simplest and most successful spintronic device currently used, the spin valve. A spin valve is a device consisting of two or more conducting magnetic materials, which alternates its electrical resistance depending on the alignment of the magnetic layers. The magnetic layers of the device align "up" or "down" depending on an applied external magnetic field. Layers are made of two materials with different hysteresis curves so that, as the magnetic field changes, one layer changes polarity, the “soft layer”, while the other “hard” layer keeps its polarity. In the figures below, the top layer is soft and the bottom layer is hard. When a magnetic layer is polarized, the unpaired carrier electrons align their spins to the external magnetic field. When a potential exists across a spin valve, the spin-polarized electrons keep their spin alignment as they move through the device. If these electrons encounter a material with a magnetic field pointing in the opposite direction, they have to flip spins to find an empty energy state in the new material. This flip requires extra energy which causes the device to have a higher resistance than when the magnetic materials are polarized in the same direction. 4 Spin valves are often used in hard disk drives to read information in the form of small magnetic fields above the surface of the disk. Spin Transfer When spin-polarized current is passed into a nanoscale magnet, it has the tendency to transfer some of its spin angular momentum into the magnet. This spin transfer arises from the “repolarization” of the current in the presence of the new external field. This applies a relatively large torque to the magnetization within the magnet. This torque will pump enough energy to the nanomagnet for its magnetic moment to precess. It will begin to move at microwave symmetry frequencies axis with around ever the increasing amplitude, and if the damping force is small, the magnetization will reverse its orientation. This is an unwanted effect in devices such as the spin valve in HDD’s since it obstructs the ability to measure the state of the valve without disturbing it. However this quantum effect is a promising tool in the development of nonvolatile magnetoresistive random access memory (MRAM). Applications MRAM utilizes magnetic storage elements instead of the electric ones used in conventional RAM devices. The elements are formed from two ferromagnetic plates, each holding a magnetic field, separated by a thin insulating layer. One plate is a permanent magnet with a constant polarity and the other plate’s polarity will change with an external field. Data is read by passing a current through an associated transistor through this “cell” and measuring the resistance. If the plates’ polarities are parallel, the resistance will be low, thus corresponding to “0”. If they are antiparallel, the resistance will be higher and the data read to be “1”. Data is written to the cells by passing a current through write lines above and below the cell, inducing a writing field. Conventional MRAM needs a fairly substantial current to produce the fields used in writing to the layers so, at small physical dimensions, many feel that MRAM will have problems at high densities. The spin torque transfer writing technology is a method in which data is written by aligning the spin direction of the electrons flowing through a tunneling magnetoresistance element. Data writing is performed by using a spin-polarized current with the electrons having the same spin direction. The spin torque transfer MRAM avoids the necessity of high power to generate the writing fields, requiring write currents not much larger than the read currents. Spin torque transfer technology has the potential to make possible MRAM devices combining low current requirements and reduced cost. MRAM promises higher density memory chips that retain information even when power is switched off. Based upon arrays of spin transistors, MRAM will combine processing and storage hence removing the distinction between working memory, such as RAM, and longer-term storage, like HDD’s. This technology relies on the spin transistor, another spintronic device still in development. The conventional transistor, invented in the 1940’s, is used in every modern complex electronic device; there are hundreds of millions on a microprocessing chip in today’s computers. By varying the bias voltages across the semiconductor junctions of these transistors, they are able to act as “switches” controlling the flow of current. Spin transistors would allow control of the quantum spin of the electrons, not just the charge current of the electrons. The diagram to the right depicts the Datta-Das spin transistor, the first first spintronic device to be proposed for fabrication in the metal-oxide-semiconductor geometry. Spin Semiconductors For spin transistors to perform the desired functions, unique materials must be developed that display both magnetic and electric properties. They must display ferromagnetic properties, able to control the spin polarization of the currents, yet still retain their useful semiconductor properties. While many traditional magnetic materials, such as magnetite, are also semiconductors, materials scientists generally predict that magnetic semiconductors will only find widespread use if they are similar to well-developed conventional semiconductor materials. Researchers have recently focused on developing dilute magnetic semiconductors. DMS is a term given to materials that are based on traditional semiconductors, but are doped with transition metals with ferromagnetic properties. Commonly, manganese, an element with magnetic properties, is doped into a semiconductor like indium arsenide or gallium arsenide. “Ideally, each Mn dopant atom represents an acceptor that introduces a local spin and a hole carrier.”6 Important research in this field has been going on right here in the University of Tennessee. Hanno Weitering and James Thompson have successfully grown thin films with magnetic and semiconducting properties. Using molecular beam epitaxy, MnxGe1-x films were grown on undoped Ge(100) substrates. In a similar experiment, epitaxial growth is used to form a thin film of Mn5Ge3 on Ge(111). According to Weitering et al., “The crystalline quality, surface topography, and thermal stability of the films indicate the possibility of growing epitaxial Ge on top of Mn5Ge3 so that epitaxial trilayers or ‘spin valves’ and perhaps even multilayer structures can be fabricated for spintronics research and applications.”5 Research projects such as these are helping to realize the practical applications of magnetic semiconductors and spintronics. Conclusion The spin of the electron has attracted renewed interest recently because it promises a wide variety of new devices that combine logic, storage and sensor applications. This interest arises from the looming problem of exhausting the fundamental physical limits of conventional electronics. Moreover, these "spintronic" devices might lead to quantum computers and quantum communication based on electronic solid-state devices, thus changing the perspective of information technology in the 21st century. 1. “Cramming More Components onto Integrated Circuits”. Electronics Magazine. Volume 38, Number 8, April 19, 1965. 2. “Spintronics”. Physics World. April 2002. 3. “Spintronics”. Scientific American. June 2002. 4. “Renesas, Grandis to Collaborate on Development of 65 nm MRAM Employing Spin Torque Transfer”. PhysOrg. 15:23, published December 1, 2005. 5. C. Zeng, S. Erwin, L. Feldman, A. Li, R. Jin, Y. Song, J. Thompson, H. Weitering, Appl. Phys. Lett. 83, 24, (2003). 6. A. Li, J. Wendelken, J. Shen, L. Feldman, F. Thompson, H. Weitering, Phys. Rev. B 72 (2005). 7. A. Li, J. Shen, J. Thompson, H. Weitering, Appl. Phys. Lett. 86 (2005). 8. I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76 (2004). 9. J. Fabian, I. Zutic, S. Das Sarma, Phys. Rev. B 66 (2002). 10. www.wikipedia.org